Measuring cuvettes of this class are used above all for stationary gas-measuring devices, especially so-called gas detectors, with which plants intended for the delivery and processing of combustible substances are secured. Such stationary gas-measuring devices usually measure the local gas concentration at the site of installation of the device. If these gas-measuring devices are used to monitor a process gas present in a closed process or to monitor the gas concentration at a certain point (so-called point detection), they usually have a measuring cuvette, through which the process gas to be analyzed is specifically sent.
To guarantee the monitoring of gas concentrations in closed processes, the process gas to be analyzed is first removed in these cases from the process proper and then introduced into the measuring cuvette. A suitable tubing or hose line, which is fastened at special ports of the measuring cuvette, is used for feeding and removing the process gas.
If a measuring cuvette of this class is used in a gas-measuring device, this device has a mount, within which the measuring cuvette is positioned and fixed. Such a gas-measuring device, especially gas detector, usually has, furthermore, at least one radiation source as well as a detector, between which the ray path with the measuring cuvette arranged is located. In any case, the measuring cuvette is arranged in the optical path between the radiation source and the optical detector such that the emitted radiation passes through the process gas when passing through the measuring cuvette. After the radiation has passed through the sample space of the measuring cuvette for the first time, it usually falls on a reflector element, especially a mirror, is reflected here in the direction of the measuring cuvette and passes through the sample space in an at least nearly opposite direction for a second time. Since the sample space of the measuring cuvette is passed through by the light beam twice, the effective length of the measuring section is doubled and more accurate measurement is thus made possible.
While the radiation, whose wavelength is often in the infrared range, is passing through the sample space of the measuring cuvette filled with process gas, the property, especially the intensity and/or wavelength, of the radiation changes because of a partial absorption of the radiation by the process gas. After passing through the sample space of the measuring cuvette, the radiation is focused on a radiation detector in a suitable manner and the detected radiation is analyzed based on a comparison with the radiation emitted originally.
A corresponding optical gas analyzer with a pyroelectric detector element is known from DE 195 43 105 C2. The gas analyzer has a sensor, which determines the percentage of special gas components in a gas sample based on an analysis of the absorption of infrared radiation in the sample gas. The technical solution described is characterized above all by a special mounting of the detector, which embodies a preferred elastic mounting based on the use of an elastomer. An advantageous vibration decoupling and, as a result, broad-band analyzability of the measured signal are made possible based on the special mounting.
Furthermore, a spectroscopic sensor, in which the infrared radiation source and the detector are fastened and contacted on a circuit board, e.g., a printed circuit board, is known from DE 10 2009 027 136 A1. The absorption section is designed here as an interior space of a cylindrical reflector device, which is likewise fastened to the circuit board and has a reflecting inner surface. Direct transmission of radiation from the infrared radiation source to the detector, i.e., without reflection at the reflector device, is prevented by means of a diaphragm provided herefor. Thus, the sensor described is characterized in that the reflector is located within the sample chamber of the measuring cuvette.
It is common to all prior-art measuring cuvettes that these have a sample space for the process gas to be measured as well as at least one window element, via which the electromagnetic radiation is coupled into the sample space from a radiation source. To obtain the longest possible measuring section within the sample space filled with the process gas to be monitored, suitable reflector devices are provided either within the sample space or outside thereof. The prior-art measuring cuvettes thus enclose a defined gas volume and have optical accesses in order to make it possible for the electromagnetic radiation emitted by the radiation source to pass through the gas to be analyzed. The optical accesses become contaminated during the operation due to dirt particles present in the ambient air or in the gas to be analyzed. This leads to losses of signal and compromises the measurement of the gas.
The optically transparent element is permanently bonded into the measuring cuvette in the prior-art measuring cuvettes. This compromises the accessibility and thus makes it difficult to clean the optical accesses. Since sapphire disks are often arranged in the optical accesses, replacing the measuring cuvette in question solely because of contamination is, furthermore, undesired.